The presence of vast world-wide deposits of low-ranked coals continues to create interest in processes for coal liquefaction. Because low-ranked coal-derived liquids must compete in the marketplace against other, more easily obtained liquid petroleum products, energy producers continue to search for integrated low-cost liquefaction processes which can provide competitively-priced liquid fuels.
Many schemes for converting coal to hydrogen-rich liquids require hydrogenation in the presence of 2000 to 3000 psig of hydrogen gas, often in ebullated, supported-catalyst hydrotreating reactors. These schemes frequently are not favored because they require relatively high capital and operating expenditures.
One way to reduce the cost of coal-derived liquids is to conduct the liquefaction process at relatively low operating temperatures and pressures and in the presence of a hydrogen donor other than high pressure hydrogen. These processes often can be conducted in relatively inexpensive, low pressure stirred or mixed reactors rather than the ebullated bed reactors typically employed in high pressure hydrogen liquefaction processes. One liquefaction reaction suitable for use in such processes is reacting crushed coal in the presence of an alkali metal base, an alcohol and a catalyst to liquefy the coal and to hydrogenate, and in some cases alkylate, the coal-derived liquids. Laboratory explorations of these processes have been disclosed by Mondragon et al. in Fuel, Vol. 61, November 1982, pages 1131-1134; Vol. 63, May 1984, pages 579-585; and Vol. 64, June 1985, pages 767-771, and by Ozaki et al. in Fuel Processing Technology, Vol. 14, pages 145-153 (1986).
Other workers have disclosed the solubilization of coal in methanol and sodium hydroxide in the absence of a dissolution catalyst. For example, in Koks, Smole, Gaz 31(2) 23-6 (1986), Salbut et al. disclosed a process in which coal pre-extracted by a benzene/ethanol mixture was liquefied in methanol and sodium hydroxide at about 325 degrees Centigrade.
Other workers have attempted to enhance the alcohol/base liquefaction of coal by providing a coal pre-treatment step. For example, in Fuel Processing Technology, Vol. 19, pages 287-292 (1988), Salbut et al. disclosed a process in which a performic acid oxidation step precedes a methanol/sodium hydroxide liquefaction step. Salbut noted that in each example therein, the oxidized coal produced a lower liquefaction yield and contained an increased number of carboxyl and hydroxyl groups which had to be eliminated by subsequent hydrogenation.
Both Salbut's reduced liquefaction yield and increased hydrogenation requirements suggest that performic acid pre-treatment is not an optimal pre-treatment step for alcohol/base liquefaction processes. Salbut's process also is not preferred because the high levels of carbonyl groups present in the pretreated coal increase the conversion of sodium hydroxide to less effective liquefaction agents such as sodium carbonate and sodium bicarbonate. Finally, because Salbut's process appears to oxidize minerals present in the coal to highly oxidized water-insoluble compounds, his pre-treatment is not well suited to recovering solid pre-treated coal apart from insoluble minerals which, if not separated from the coal, can hinder the effectiveness of downstream process steps such as reagent reclamation.
Other coal pre-treatment schemes such as those disclosed in U.S. Pat. No. 4,161,440 pre-treat coal with a sulfur oxide to form insoluble mineral salts that remain stable during liquefaction. In similar processes like those disclosed in U.S. Pat. No. 4,304,655, an oxidizing agent such as oxygen is added during the pretreatment step. While the insoluble salts formed by these pre-treatment steps may reduce reactor scaling under high pressure hydrogen liquefaction conditions, these processes are not preferred for use with a base/alcohol liquefaction process because oxidation of the coal produces additional carbonyl groups in the coal. These additional carbonyl groups can hinder the liquefaction process because they can convert the alkali metal hydroxide liquefaction reagent to less effective carbonate and bicarbonate forms. These processes also are not preferred because they introduce insoluble mineral matter into the liquefaction reactor, thereby potentially interfering with the reclamation of alkali metal meterial removed from the reactor.
Thus, a need exists for an improved low severity alcohol/base liquefaction process having a coal pre-treatment step which can reduce the carboxyl content of the coal prior to the liquefaction step. The process preferably should provide for high product yields and product quality while at the same time facilitating the reclamation of unconsumed or reclaimable base and alcohol liquefaction reagents.
Our commonly assigned U.S. application Ser. No. 07/689,192 discloses a coal liquefaction process in which coal undergoes an initial decarboxylation step in the presence of hot, liquid water and sulfurous acid. It has now been found that this hot sulfurous acid pre-treatment step provides unexpected advantages when used as part of an integrated alcohol/base liquefaction process.